Optical networks are increasingly being used in many industries, most notably telecommunications and computer networks. Such networks use light as an optical signal to transmit data, typically over a glass fiber. For example, a laser can create a beam of light onto which a modulator encodes data to form an optical signal. The light may be transported via a fiber optic cable to a destination of interest. Fibers often carry multiple wavelengths or bands of wavelengths of light simultaneously—each one encoded with its own data stream. These wavelengths can be combined by a device called a multiplexer and placed on the fiber. For example, Dense Wavelength Division Multiplexing (DWDM) is a fiber-optic transmission technique which permits multiplexing many optical signals of different wavelengths onto a single fiber.
On the receiving end, the laser light is typically demultiplexed, that is, split into individual optical signals by wavelength or band of wavelengths. Each optical signal is often routed to a separate photodetector, which converts the optical signal into an electric signal, which can then be routed to host logic for processing. Thus a glass fiber may accommodate many optical channels, each channel carrying an optical signal which may be separated from the other optical signals of the fiber and processed. Optical networks may also use other items such as mirrors, splitters, and switches to manipulate the light, or add or drop an optical signal at various locations.
One device which can facilitate Dense Wavelength Division Multiplexing is an arrayed waveguide grating (AWG) module that is capable of handling multiple wavelengths. An AWG is typically based on planar lightwave circuit (PLC) technology suitable for mass production. Modules can be relatively thin, such as less than 10 mm and may be sealed in a package.
FIG. 1 shows a schematic diagram of a known AWG demultiplexer 10 which is formed on a substrate 12. A beam of light represented by arrow 14, comprises a plurality of multiplexed optical signals which are carried on a plurality of wavelengths λ1, λ2 . . . λn. The beam 14 is applied to an input waveguide of an array 16 of input waveguides of the AWG demultiplexer 10 which carries the beam to an input slab waveguide 18. The beam 14 is diffracted into the input slab waveguide 18 which spreads the optical signals of the beam 14. The spread optical signals are input to a plurality of inputs 20 of an array 22 of waveguides 24, each of which propagates an input optical signal along the length of the particular waveguide 24, to an output 26.
Typically, the waveguides 24 are arranged in the array 22 so that the length from input 20 to the associated output 26 for each waveguide 24, differs from the adjacent waveguide 24 by a length differential, such as, d1. As a consequence, a phase offset corresponding to the length differential d1 may be imposed on the optical signal being propagated by a particular waveguide 22, relative to the phase of the optical signal propagated by an adjacent waveguide 24.
The optical signals passing through the array 22 of waveguides 24, are again spread by diffraction by an output slab waveguide 30 coupled to the arrayed waveguide outputs 26. However, due to the mutual interference between the optical signals from each waveguide 24, caused by the phase difference between adjacent optical signals, the wavefronts of the optical signals may be diffracted in a substantially uniform direction as a function of the wavelength of the optical signals. Accordingly the optical signals of a common wavelength can be focused at substantially the same position at the output side 32 of the output slab waveguide 30. An input 34 of a waveguide 36 may be positioned at each output position of the output slab waveguide 30. Thus, the optical signals of differing wavelengths can be focused at different positions 34 on the output side 32 of the output slab waveguide 30 to permit each optical signal to be sent to a different output waveguide 36 of an array 38 of output waveguides, as a function of wavelength. In this manner, the optical output signals carried by wavelengths λ1, λ2 . . . λn may be demultiplexed, that is, extracted from the mulitplexed optical input signal 14.
The AWG demultiplexer 10 may also be operated as a multiplexer. A plurality of input optical signals as represented by the wavelengths λ1, λ2 . . . λn may be input into the associated waveguides 36. The AWG operated in reverse, combines the various optical signals into a multiplexed beam which is output from one of the waveguides 16.
The operation of an AWG may be temperature dependent. Accordingly, an AWG device may include a temperature control element to facilitate achieving an appropriate multiplexing or demultiplexing function.
Each waveguide 36 provides a separate output channel O1, O2 . . . On. The quantity of light passing through each output channel may be measured as a function of wavelength and polarization. Known devices for measuring light output as a function of wavelength include the JDS Uniphase SWS. FIG. 2 shows a schematic example of the wavelength response of a known AWG for each of a plurality of output channels O1, O2 . . . On. Thus, for example, the response of the AWG demultiplexer 10 to a particular light beam with a particular polarization at output channel O1 has a peak indicated at 40 at a particular wavelength of the input light beam.
Many AWG's have a polarization dependent wavelength response (PDW), that is, the wavelength at which the peak response occurs may differ, depending upon the polarization of the light beam. One known polarization is referred to as transverse electric (TE) in which the electric field vector is normal to the direction of propagation. Thus, in fiber optics, a polarized beam having a TE polarization has an electric field vector that passes through the optical axis of an optical fiber. Conversely, a polarization referred to as transverse magnetic (TM) is one in which the magnetic field vector is normal to the direction of propagation. Thus, in fiber optics, a polarized beam having a TM polarization has a magnetic field vector that passes through the optical axis of an optical fiber. When AWG's are measured using the JDS Uniphase SWS system, the response is measured as a function of the polarizations that provide maximum and minimum loss, which are not necessarily TE and TM. However it is frequently assumed that the measured polarizations of maximum and minimum loss are TE and TM (although which of these has maximum loss and which has minimum is often not known). This assumption is approximately true in many cases.
In FIG. 2, a polarization dependent wavelength response for a light beam having a first polarization is represented by a solid line. The polarization dependent wavelength response for a light beam having a second polarization, different from the first polarization, is represented by a dashed line. Thus, for example, the response of the AWG demultiplexer 10 to a light beam having the first polarization at output channel O1 has a peak indicated at 40 at a particular wavelength of the input light beam. The response of the AWG demultiplexer 10 to a light beam having the second polarization at output channel O1 has a peak indicated at 42 at a particular wavelength of the input light beam.
As indicated in FIG. 2, the peak responses for a light beam having one polarization may be at a somewhat lower wavelength than the peak response for a light beam having a different polarization. In measuring these polarization dependent responses, many instruments may not readily provide sufficient information to determine which polarization, TE or TM or some combination thereof, resulted in the response having the lower (or higher) peak wavelength. One known technique for identifying the polarizations of the polarization dependent responses is to provide as an input a light beam having a known polarization.